Battery energy storage systems earn revenues by dispatching energy to the grid or by providing Ancillary Services - but only when they are online and operational.
Availability refers to how much and how often a battery can export power. But how available are battery energy storage systems in ERCOT?
Since January 2021, battery energy storage systems in ERCOT have had an average availability of 93%. This means that, at any given time, we can expect 93% of the total rated power (MW) of batteries in ERCOT to be online and available.
To put this into perspective, the average availability of thermal generators in ERCOT is 76%.
Also, battery availability does not experience the same consistency of seasonal fluctuation as thermal availability. (In the shoulder months of early spring and late fall, the thermal generators in ERCOT typically schedule maintenance.)
So, where have these numbers come from?
How we measure availability
Here, we have measured availability using each resource’s High Sustained Limit (HSL). The HSL is a real-time estimate of a resource’s output capability. For every battery energy storage system in ERCOT, the Qualified Scheduling Entity (QSE) must provide a telemetered HSL to ERCOT for every five-minute SCED interval. This HSL reflects the capacity that the battery is capable of sustaining for the next five minutes.
Let’s use a 10 MW system as an example:
- If it can sustain 10 MW of output for the next five minutes, its HSL is 10 MW. As a result, its availability is 100% (for that five minutes).
- If it can sustain 5 MW of output for the next five minutes, its HSL is 5 MW. As a result, its availability is 50%.
- And if it can’t sustain any level of output for the next five minutes, its HSL is 0 MW. As a result, its availability is 0%.
By looking at the HSLs of a battery over time, we can measure its availability across that period. This measure also takes into account any downtime or derating.
And, when we do this for every battery in ERCOT, we can calculate the overall availability of this technology type.
As mentioned above, battery availability doesn’t exhibit the same seasonal trends as thermal generation. So, what actually affects the availability of battery energy storage systems in ERCOT?
Does temperature affect battery energy storage availability?
Thermal generators tend to operate at lower efficiencies (and, therefore, have a lower maximum power output) at higher temperatures.
But are batteries similarly affected by temperature?
In fact, there is close to zero correlation between the maximum daily temperature (in Dallas-Fort Worth) and the overall availability of battery energy storage systems on that day.
For days in the normal temperature range of 60-80 degrees Fahrenheit, average battery availability ranged between 86% and 100%.
However, at more extreme temperatures, there were very slight drops in availability.
- On days when the maximum temperature was below 35 degrees, average battery availability dropped to 90%.
- And, on days when the maximum temperature reached 105+ degrees, average battery availability dropped to 92%.
So, even at the extremes, temperature has a negligible impact on battery availability in ERCOT.
Subscribers to Modo Energy’s ERCOT products can read the rest of the article below - to understand battery availability during high-revenue opportunities, and to see how availability differed by owner.
Do wholesale energy prices impact battery availability?
Of course, battery energy storage system operators would prefer that their resources are available when high revenue opportunities occur. But does this actually happen?
On days when energy price spreads in ERCOT were below $500/MWh, there was close to zero correlation between the size of the spread itself and battery energy storage availability. Essentially, batteries do not tend to be more available when there are greater revenue opportunities.
If anything, the opposite is truer.
On days with extreme energy wholesale price spreads, battery availability actually dropped slightly. When spreads were between $4,000 and $5,000/MWh, average battery availability was 92%.
These extreme price spreads are exceptionally rare. They occur because of huge spikes in energy prices, caused by system tightness. They are typically a result of severe weather conditions and/or large generation outages.
Even during these rare events - when the ability to capture these spreads can have a huge impact on a battery’s revenue performance (e.g. in 2023, 50% of battery energy storage revenues were earned in just 13 days) - wholesale energy price spreads had little to no impact on battery availability in ERCOT.
As we can see, system-wide battery availability is not driven by external conditions or market opportunities.
So then, what does impact availability?
How did availability vary by owner?
As mentioned above, 50% of battery energy storage revenues were earned in just 13 days in 2023. As such, being available on these days was crucial for maximizing revenues. In fact, one particular system lost out on around $40,000/MW last year - around 20% of its annual income - due to unavailability (which we previously measured by Resource Status, rather than by High Sustained Limit).
As such, it is critical that batteries are available as much as possible.
And, in 2023, some owners’ portfolios were much more available than others.
Jupiter Power, Eoilian, Hunt Energy Network, Vistra Corp, and Plus Power all achieved at least 97% availability across their battery portfolios in 2023. These owners are some of the largest battery portfolios in ERCOT, with many years of experience in operating battery projects.
Ultimately, as a technology type, battery energy storage systems in ERCOT are reliably and consistently available - particularly when compared to thermal generators - regardless of temperature or revenue opportunity.